Add ADR for "entry(s)"

[koppor]

Add ADR on pluralization.

Risen by #12124 (comment).

This also replaces "entry(ies)" by "entry(s)", which read better IMHO.

Seeign Ukrainian listed at first example at https://cldr.unicode.org/index/cldr-spec/plural-rules, I ask @InAnYan about his oppinion here.

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[tobiasdiez]

A couple of days I actually looked into this myself and found https://support.crowdin.com/icu-message-syntax/#plural. Can we not use this?